Field of the Invention
The present invention relates to a casing of a light scanning apparatus included in a digital copying machine, a laser beam printer, a facsimile machine, or other electrophotographic image forming apparatus, and to a light scanning apparatus and an image forming apparatus.
Description of the Related Art
Hitherto, a light scanning apparatus used in an electrophotographic image forming apparatus forms light spots on a surface to be scanned by deflecting a light beam emitted from a light source with a deflection device which includes a rotary polygon mirror and converging the light beam toward the surface to be scanned with an imaging optical system. The light scanning apparatus scans the surface to be scanned with the formed light spots to form a latent image on the surface to be scanned. The deflection device and the imaging optical system including lenses and reflective mirrors are mounted inside a predetermined casing (hereinafter referred to as “optical box”) of the light scanning apparatus. In the light scanning apparatus, a rotary shaft of a motor configured to rotate the rotary polygon mirror may be inclined (hereinafter referred to as “shaft inclination”) with respect to a design rotary-shaft mounting angle depending on a manufacturing accuracy. When the shaft inclination of the rotary polygon mirror occurs, the position and the angle at which the light beam deflected by the deflection device enters the lens of the imaging optical system are equally deviated from the design values. Therefore, light beam spots each having an irregular shape are formed on the surface to be scanned, leading to reduction in imaging performance and deterioration in image quality.
FIG. 19A and FIG. 19B are illustrations of an example of a configuration of a deflection device of a comparative example. In FIG. 19A and FIG. 19B, an angle of a rotary shaft 804 with respect to a drive board 801 varies depending on a manufacturing accuracy and a caulking accuracy of a member for mounting a bearing 802 to the drive board 801. When the drive board 801 warps, the rotary shaft 804 having a rotary polygon mirror 805 mounted thereon is similarly inclined to cause shaft inclination. In this manner, reflective surfaces of the rotary polygon mirror 805 are equally inclined, and a reflection angle in a sub-scanning direction of reflected light is deviated from an ideal position, resulting in reduction in optical characteristics. Further, it is difficult to process mounting seat surfaces 809, 810, 811, and 812 for the drive board 801, which are formed on an optical box 807, to have ideal flat surfaces in a strict sense. Therefore, the deviation from the ideal flat surfaces also appears as the shaft inclination of the rotary polygon mirror 805.
In the light scanning apparatus, as an item that influences the productivity of the image forming apparatus (speed to output images in a predetermined time period), there may be given the number of revolutions of the motor configured to drive the rotary polygon mirror in addition to the number of light sources and the number of surfaces of the rotary polygon mirror. Therefore, there has been a demand for increase in number of revolutions of the motor in order to increase the productivity of the image forming apparatus. Along therewith, there are problems of motor durability, and noise and vibration caused by the motor. Therefore, for example, in Japanese Patent Application Laid-Open No. 2013-54082, the following light scanning apparatus has been proposed. That is, dedicated deflection devices that differ in durability or rotation control method depending on the number of revolutions of the motor are prepared, and the dedicated deflection devices are allowed to be mounted to a common optical box.
When deflection devices including motors having different numbers of revolutions are allowed to be mounted to a common optical box, the following three problems arise. Description below is made assuming that a common optical system, that is, the common rotary polygon mirror 805 is used to use the common optical box. The first problem resides in a difference in shaft diameter of a bearing due to a difference in bearing structure of the motor. For example, magnitude relationships of a bearing diameter and a bearing length between motors 803a and 803b are shown in FIG. 20 and FIG. 21. In this case, a hole diameter of a positioning hole 808 of the optical box 807 has a step difference as illustrated in FIG. 20 and FIG. 21. In this manner, the motors 803a and 803b having different shaft diameters can be mounted to the common optical box 807. When the above-mentioned magnitude relationships are not satisfied, the optical box 807 cannot be shared.
Next, the second problem resides in a difference in height from a back surface of the drive board to the rotary polygon mirror 805 (hereinafter referred to as “mirror height”). When there is a difference in mirror height due to a difference in motor structure or other reasons, the mirror heights are required to be set to the same height in order to allow usage of the two motors in the common optical box. The motors 803a and 803b have different mirror heights 817a and 817b as illustrated in FIG. 22A and FIG. 22B, respectively, and a magnitude relationship between the mirror heights 817a and 817b satisfies 817a>817b. In this case, in order to set the heights from the optical box 807 to the rotary polygon mirror 805 to the same height, heights of respective bosses 809a to 812a and 809b to 812b formed on the optical box 807 are required to satisfy a relationship represented by the following expression.809a=810a=811a=812a<809b=810b=811b=812b
The third problem resides in differences in size of the drive board and screw hole position. When the area of the drive board differs depending on a difference in scale of the drive circuit due to the difference in number of revolutions of the motor, the motors also have a difference in screw hole position in accordance therewith. In this case, in order to allow usage of the two motors in the common optical box 807, as illustrated in FIG. 22A, FIG. 22B, FIG. 23A, and FIG. 23B, the size of the drive board is required to satisfy the following magnitude relationship. That is, the sizes of drive substrates of a drive board 801a for the motor 803a and a drive board 801b for the motor 803b are required to satisfy a magnitude relationship represented by the following expression.801a<801bWhen the above-mentioned magnitude relationships, which are determined based on the mirror height 817 and the size of the drive board 801, are not satisfied, the optical box 807 cannot be shared. Details of FIG. 19A and FIG. 19B to FIG. 22A and FIG. 22B are described later.
The problems that may arise when the deflection devices including motors having different numbers of revolutions are allowed to be mounted to the common optical box have been described above. It is also conceivable to prepare different optical boxes depending on the motors having different specifications. However, in terms of reduction in cost and type of the optical box, a great advantage can be obtained when usage of a common optical box is allowed. In Japanese Patent Application Laid-Open No. 2013-54082, positioning to a common optical box is allowed by changing presence or absence of a connection plate. In this method, however, the connection plate is interposed between the optical box and the motor, and hence an unintended stress may be applied to the back surface of the drive board due to warpage of the connection plate or a difference in thickness of the connection plate. That is, this method affects a posture of the motor, in particular, the shaft inclination accuracy. The shaft inclination of the motor causes reduction in performance of imaging on the surface to be scanned, which directly leads to deterioration in image quality. Therefore, the connection plate is shaped to be deformable by forming a cutout in a coupling portion of the connection plate, but the thickness of the connection plate may be increased depending on the difference in mirror height of the rotary polygon mirror between the deflection devices having different specifications. In this case, even when the connection plate is shaped to be deformable with the cutout, the bending rigidity of the connection plate is increased. Thus, the stress to be applied to the back surface of the drive board cannot be reduced, and the factor that causes reduction in shaft inclination accuracy cannot be eliminated.